Total internal reflection thermally compensated rod laser

ABSTRACT

A total-internal-reflection thermally compensated rod laser including a lasing rod composed of crystalline or glass material doped with at least one lasing ion. The rod has an optical axis, an axially extending, substantially optically flat, exterior surface, and a substantially optically flat conical surface at each end. Both conical end surfaces are coaxial with the optical axis and has a truncated tip. The first end has a convex surface and the second end has a concave surface. The rod has a geometry, including a diameter and a length, selected to provide substantially no net depolarization of an incident beam.

This application claims benefit of Provisional Application Ser. No.60/056,620 filed Aug. 20, 1997.

BACKGROUND OF THE INVENTION

This invention relates generally to solid state lasing elements. Moreparticularly, the present invention relates to total internal reflectionrod-shaped lasing elements.

Heat is generated in solid-state lasing elements as part of the opticalpumping process in which a flashlamp, diode-array, or other sourcesexcite the lasing ions doped into solid-state laser materials such asNd:YAG, Yb:YAG, Er:YAG, and many others. See D. C. Brown, "Heat,Fluorescence, and Stimulated-Emission Power Densities and Fractions inNd:YAG," IEEE Journal of Quantum Electronics, Volume 34, pages 560-572,1998. For very low repetition-rate operation, ample time between pulsesallows the laser material to return to thermal equilibrium and nodeleterious effects on laser performance occur. When, however, theaverage pumping and laser output power becomes significant, thermaleffects begin to play a significant role and the effect on laserefficiency and operation can be severe.

For rod geometry lasers where the lasing material is a right circularcylinder, attempts to remove the heat generated usually involve coolingthe rod 10 along its barrel with water 12 as shown in FIG. 1. Underideal circumstances in which the pumping of the rod is absolutelyuniform and heat is removed in a symmetric and uniform way along the rodbarrel, and rod materials parameters independent of temperature, atemperature distribution is established in the rod transverse dimensionwhich can be calculated analytically. It has been found that theradially dependent temperature distribution varies quadratically withthe radial coordinate r. See W. Koechner, "Solid-State LaserEngineering," 4th Edition, Springer-Verlag (1996). In essence the heatis distributed across the rod such the highest temperature is at thecenter of the rod and the lowest temperature is at the rod barrel.

In common laser materials such as Nd:YAG, the index of refraction is afunction of temperature and in fact increases with temperature. Hence,there exists a radially varying index of refraction distribution in therod that follows the temperature distribution wherein the temperature islargest in the center and lowest at the rod edge or barrel. Because ofthe radially varying index of refraction, light propagation through therod is affected. Because light travels slower as the refractive indexincreases (at a speed v=c/n where c is the vacuum speed of light and nthe index of refraction), it travels most slowly along the rod axis andfastest at the rod edge.

As shown in FIG. 1, an incident plane wave 14 with flat phase frontlooks curved 16 after propagating through the rod since the center phaseis retarded with respect to the rod edge. It can be shown that ideallythe rod functions like a thick lens and that, in the presence of strongthermal effects, the beam exiting the rod is focused. There are two suchfoci in a laser rod such as Nd:YAG. The tangential and radialpolarizations have separate foci that do not overlap, and the focallengths can be calculated exactly. See W. Koechner, "Solid-State LaserEngineering," 4th Edition, Springer-Verlag (1996). An associatedphenomena, birefringence, accompanies strong thermal focusing. Forlinearly polarized input to the amplifier, a phase difference isaccumulated between the radial and tangential components of thepolarization. The magnitude of the phase difference depends upon thetype of lasing material used, the thermal loading of the rod, and thelocation in the aperture. Such birefringence is detrimental in rodlasers that must use an intracavity polarizer, such as some Q-switchedlasers, since the output from the rod is elliptically polarized andsignificant losses can result from the polarizer. See W. Koechner,"Solid-State Laser Engineering," 4th Edition, Springer-Verlag (1996).

If the rod is pumped harder and harder the thermally-induced focusingbecomes stronger and stronger. Laser resonators that use rod amplifiersare then particularly susceptible to this phenomena. As pumping averagepower is increased, the rod focuses more strongly. This results inchanges in the mode-structure or content of the output beam, acontinuous change in the output beam quality, and eventually instabilityof the resonator thereby causing it to stop lasing. In some resonators,beam quality improves with operating average power until the best beamquality is achieved. Pumping beyond this single point then results indegradation of beam quality.

A number of attempts to reduce or eliminate the thermal focusing havebeen implemented. For example, a slab 18 (FIG. 2) may be used ratherthan a rod of laser material such as disclosed in U.S. Pat. No.3,633,126. The beam is totally-internally-reflected back and forthbetween the slab faces 20 through which pumping is incident. A flow ofwater 12 along the slab TIR faces cools the laser crystal (see FIG. 2).The medium used to construct the slab is usually a crystalline or glassmaterial although liquids have also been used. The slab ends 22 areusually either uncoated and cut at or near Brewster's angle so thatthere are no reflective losses, or anti-reflective (AR) coated for somearbitrary angle of incidence on the faces. The slab is usually thin,typically 5-7 mm, and long. It is usually designed to operate with aneven number of bounces off the slab faces through which pump light isdelivered to the lasing material. The slab is either transversely cooledwith typically water, a water/ethylene glycol mixture, or gas either asshown in FIG. 2 or most often using longitudinal cooling along the slabfaces. Conduction cooling has also been used. The pump faces and endfaces must be optically flat and accurately cut and oriented; bouncingoff the slab faces is via TIR so 100% reflection is obtained.

This technology is referred to in the literature as a face-pumpedtotal-internal-reflection (TIR) laser or slab laser. Other more recentvariants of that concept are the hex laser concept of U.S. Pat. No.4,740,983, and the edge-screw laser of U.S. Pat. No. 4,912,713. Slablaser technology has been developed to the point where kilowatt levelsof power can be achieved with excellent output beam-quality that remainsconstant in the TIR direction. The zig-zag path of the beam back andforth within the laser slab results in internal compensation for thermaleffects since each ray incident upon the input aperture experiencesapproximately the same total thermal environment. Thus, first-orderthermal focusing does not occur in a TIR slab laser.

U.S. Pat. Nos. 3,810,040 and 3,810,041 disclose liquid cooled slablasers. U.S. Pat. No. 3,679,999 discloses a slab laser which isconduction cooled with gas. The main difficulty with the slab laser isthat the beam-quality does not remain constant in the directionorthogonal to the TIR direction, or the transverse direction. Manyattempts have been made to rectify this situation. However, while thethermal effects may be reduced by various techniques, the outputbeam-quality and mode-structure remain functions of the average power.When the beam is propagated and used for certain processes, such aspercussion drilling, the spot remains constant in size in the TIRdirection but varies in the transverse dimension, thereby leading toprocessing effects that are average power dependent. For manyapplications this situation is intolerable.

Polygonal rod elements which do not employ TIR to internallyself-compensate thermal focusing are disclosed in U.S. Pat. No.5,432,811. To eliminate the slab's lack of thermal compensation in thetransverse direction, "hex" TIR laser elements of U.S. Pat. No.4,740,983 and square rods of U.S. Pat. No. 4,912,713 totally internallyreflect the beam simultaneously in two dimensions. The TIR rod laserelement described here has the advantage that it can be used as areplacement for conventional laser rods and remains "in-line" with theresonator optical axis. Conventional laser rods are the most commonlasing elements and are pumped typically as shown in FIGS. 3 and 4 whereflashlamp and diode-pumping schemes are shown. The rod is in all casessurrounded by a flowtube that encloses an annulus of flowing water orother fluid used to cool the rod.

SUMMARY OF THE INVENTION

Briefly stated, the invention in a preferred form is atotal-internal-reflection thermally compensated rod laser which includesa lasing rod composed of crystalline or glass material doped with atleast one lasing ion. The rod has an optical axis and an axiallyextending, substantially optically flat, exterior surface. Each end ofthe rod has a substantially optically flat conical surface.

The conical end surfaces are coaxial with the optical axis. The firstend has a convex surface and the second end has a concave surface. Theapex or each conical end surface has a truncated tip. The conicalsurface of the first end may have an apex having an angle α which isselected to provide predictable internal lossless TIR bounces.Alternatively, α may have a value substantially equal to the complementof Brewster's angle. The rod has a geometry, including a diameter and alength, selected to provide substantially no thermal focusing of anincident beam.

In one embodiment the rod has an axially extending center column and anannular portion disposed around the column. The center column is dopedwith an active lasing ion and the annular portion is composed of a clearlaser material or an amplified spontaneous emission absorbing material.In another embodiment, the rod includes a second annular portiondisposed around the other annular portion and the column. In thisembodiment, the center column is doped with an active lasing ion, theinner annular portion is composed of a clear laser material, and theouter annular portion is composed of an amplified spontaneous emissionabsorbing material.

An annulus of fluid is disposed around the rod to actively cool it. Thefluid may be composed of water, water and ethylene glycol, gas, orcryogenic fluid.

It is an object of the invention to provide a new and improvedtotal-internal-reflective lasing rod.

It is also an object of the invention to provide a new and improvedlasing rod that is thermally compensated and has no net depolarizationon an incident beam due to TIR reflections.

Other objects and advantages of the invention will become apparent fromthe drawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood and its numerous objectsand advantages will become apparent to those skilled in the art byreference to the accompanying drawings in which:

FIG. 1 is a schematic side view of a conventional straight-through rodamplifier;

FIG. 2 is a schematic side view of a conventionaltotal-internal-reflection slab laser;

FIG. 3a is a schematic end view of a conventional classic ellipticalpump chamber,

FIG. 3b is a schematic end view of a conventional dual elliptical pumpchamber, and

FIG. 3c is a schematic end view of a conventional close-coupled diffusereflector pump chamber;

FIG. 4 is a schematic end view of a conventionaltransversely-diode-pumped laser;

FIG. 5 is a schematic side view of a total-internal-reflection thermallycompensated rod laser in accordance with the invention;

FIG. 6 is an enlarged schematic side view of the rod of FIG. 5illustrating the geometry of the beam path within the rod;

FIGS. 7a, 7b, and 7c are sectional views of alternate embodiments of therod of FIG. 5 and

FIG. 7d is a schematic end view of an alternate embodiment of the rod ofFIG. 5;

FIGS. 8a, 8b, and 8c are schematic side views of the rod of FIG. 5illustrating the unextracted regions produced by an obscured inputaperture;

FIG. 9a is a schematic side view of the rod of FIG. 5 and

FIG. 9b is a schematic end view of the rod of FIG. 5 illustrating thefocusing of rays on the rod optical axis;

FIG. 10 is a schematic end view of a total-internal-reflection thermallycompensated rod laser in accordance with the invention having conicalfaces cut at Brewster's angle illustrating the polarization componentswithin the rod;

FIG. 11 is a schematic end view of a conventional rod illustrating thepolarization components within the rod;

FIG. 12 is a schematic end view of the rod of FIG. 10 illustrating thepolarization of a ray within the rod;

FIG. 13 is a graph plotting the phase delay versus the angle ofincidence for the rod of FIG. 10; and

FIG. 14 is a schematic side view of a resonator including the rod ofFIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

The invention described here significantly improves on the conventionallaser rod geometry by total-internal-reflecting the laser beam 24traversing through the rod 26 as shown in FIG. 5. The conical ends 28,30, which correspond to the tilted ends 22, cooperate in a slab laser toreflect the beam back and forth in a zig-zag fashion through the thermalgradient in a controlled and precise way. The conical ends 28, 30 arepreferably congruent, matched convex and concave optical structures.Other than the two conical ends 28, 30 and a prescribed rod length, thelaser rod is configured similarly to a conventional laser rod. The rodis composed of crystalline or glass material doped with at least onelasing ion.

By orienting the conical surfaces 28, 30 as shown in FIG. 5, with bothcone apexes 32, 34 facing in the same direction, it can be shown thateach ray 24 incident on the face 32 experiences the same total thermalenvironment, and to first-order approximation, the wavefront traversingthe zig-zag rod experiences no thermal focusing effects. The same is nottrue if the apexes face in opposite directions; and in addition, withthat orientation, the beam is turned "inside-out". As with the slablaser, the conical surface can be oriented at Brewster's angle with nocoatings applied or at some other angle with an appropriate AR coating.

In the ideal case, each conical end 28, 30 terminates in a very sharppoint. In practice such a sharp point cannot be fabricated since the tip36 will fracture. Thus, each conical end must be truncated by a smallamount as shown in FIG. 5. Also, the sharp end 38 on the concave inward(right) end will also have to be flattened slightly. Where truncated,the surface must be ground or roughened, or the two truncated surfaces36, 38 canted with respect to one another to keep laser action frombuilding up through those faces. The rod 26 is cooled by enclosing it inan annulus and providing a flow system that provides uniform flow 40over the entire outside surface 44, with the exception of small regionson the ends where O-ring seals 42 (FIG. 14) must be placed. Thereflections on the rod barrel 44 are TIR in nature and provide 100%reflection for any polarization. The rod faces 28, 30 and barrel 44 mustbe accurately finished and be optically flat, preferably to onewavelength A or better. This can be achieved by using a recentlyperfected optical finishing technique, diamond turning.

The TIR rod laser shown in FIG. 5 can be optically-pumped exactly likeany other flashlamp or diode-pumped rod laser, as shown previously inFIGS. 3 and 4.

FIG. 6 illustrates the geometry of the laser. The conical ends 28, 30have an apex half-angle α; the rod diameter is D. The angle of incidenceθ, is related to the apex angle α by the relationship

    θ.sub.i +α=90°

N is the normal to the face, while N' is the normal to the TIR or barrelfaces. Rays are incident on the barrel faces at an angle θ≧θ_(c) whereθ_(c) is the critical angle, given by ##EQU1## where n₂ is the indexoutside the laser medium, and usually equal to the index of air (1.00)or water (1.33) and n₁ =n, the index of the laser medium.

The refractive angle θ_(r) can be found from Snell's law as

    θ.sub.r =sin.sup.-1 (sin(θ.sub.i)/n

where n is the linear refractive index at the laser wavelength. θ can beshown to be given by

    θ=90°+θ.sub.r -θ.sub.i =90°+sin.sup.-1 (sin(θ.sub.i)/n)-θ.sub.i

thus by knowing the refractive index and the angle of incidence, θ canbe calculated exactly. A special case is if the angle of incidence θ_(i)=θ_(B), where θ_(B) is Brewster's angle, given by

    θ.sub.B =tan.sup.-1 (n)

One pass through the thermal gradient (edge to center to edge) has alength associated with it of z given by

    z=D tan(θ)

and the rod length associated with N TIR reflections is then

    L=ND tan(θ)

The total path length L_(t) inside the rod is given by

    L.sub.t =ND/cos(θ)

Also shown in FIG. 6 is the pump length of the rod laser, given by

    L.sub.p =ND tan(θ)-(D/2 tan(α))

These equations allow a complete description of the rod geometry neededfor compensation.

FIG. 7 illustrates alternate embodiments of the rod of the presentinvention. In the embodiment shown in FIG. 7a, the rod comprises a core46 doped (for example with Yb or Nd) YAG surrounded by a sleeve 48 ofclear YAG (no doping). In FIG. 7b, the rod comprises a core 50 of dopedYAG surrounded by a sleeve 52 of clear YAG which is in turn surroundedby a sleeve 54 of YAG doped with Cr⁴⁺. In FIG. 7c, the rod comprises acore 56 of doped YAG surrounded by a sleeve 58 of YAG doped with Cr⁴⁺.Such composite rods may be fabricated using diffusion-bonding techniquesor with glue for example. The clear or doped annuli may be split,quartered or subdivided in other ways for ease of fabrication. Anotherimplementation is shown in FIG. 7d where strips 60 of Cr⁴⁺ :YAG arediffusion-bonded or glued along the rod barrel 62. Cr⁴⁺, is an effectiveabsorber of amplified spontaneous emission (ASE) for Nd:YAG and Yb:YAG,and is also effective in reducing or eliminating parasitic oscillationsthat thrive in rod lasers that have polished barrels and high gain. ASEand parasites are generally not a problem for continuous-wave (CW)lasers.

FIG. 8 illustrates the existence of unextracted regions within the rod.As illustrated in FIGS. 8a, 8b, and 8c, the magnitude of the unextractedregions depends on the degree to which the incident beam energy istransmitted through the input aperture. In FIG. 8a, the entire aperture64 is utilized and no unextracted regions exist. In FIG. 8b, inputaperture is obscured in three areas 66, allowing the incident beamenergy to be transmitted through only a relatively small unobscured ring68 of the input aperture 64, resulting in large unextracted regions 70.In FIG. 8c, only the center portion 72 of the input aperture 64 isobscured resulting in relatively small unextracted regions 74. The limitto the volume extraction is given by the need to have the truncated apexon the end conical regions. This can be reduced to the minimum bycareful fabrication and the use of shallow apex angles α.

FIGS. 9a and 9b illustrate an unusual feature of this laser. If weconsider a thin annulus 76 on the rod face, with radius r, we see thatall rays that are incident in the region between r and r+dr are focusedto the same point 78 on the rod optical axis 80. This is cause forconcern, particularly in pulsed lasers where the peak power andintensities can become quite large. If the bulk damage threshold isexceeded the rod material will break down in an avalanche multiplicationprocess, leading to irreversible damage along the axis 80 of the rod. Itis fortunate, however, that not all annuli on the face are focused tothe same point on the optical axis 80. Instead, it can be seen thatsuccessive annuli on the face are focused to different spots on the axis80, essentially resulting in a "smeared" focus in the z direction. Asshown in FIG. 9, making the face angle α (FIG. 6) large smears the focalregion out even further.

Diffraction and finite perturbations of the face finishing and bulkoptical index perturbations will define the transverse size of thesmeared distribution. Detailed diffraction calculations will need to beperformed if breakdown is observed in real devices. Preliminarycalculations, however, show that for CW lasers breakdown should not be aproblem. The damage threshold for the bulk of an optical material istypically 10 times that of a surface; this fact works to our advantagein this geometry since all focusing takes place in the material bulk.

A very unusual feature of this laser is the existence of polarizationeffects. If the conical faces 28, 30 of the rod 26 are cut at Brewster'sangle, only modes whose polarization is as shown in FIG. 10 (radial) canbe supported. An advantage of this configuration is that a beamtraversing the rod will suffer no thermally induced birefringence. Thisis primarily because the loss for radial polarization vectors 82 isminimal while other polarizations will suffer a great enough loss thatthey will not reach threshold in the oscillator. A TEM₀₁ mode, forexample, would oscillate using this configuration.

In general, however, most lasers 84 operate with linear polarization 86as shown In FIG. 11. The component 88 of the polarization along a radiusis referred to as the radial polarization while the perpendicularcomponent 90 to the radial is referred to as the theta component. If forany reason the two components 88, 90 become separated in phase δ, thenthe resulting polarization is referred to as elliptical. Thermal effectsin straight-through rod amplifiers result in elliptical polarization,and the amount is dependent upon the position on the rod face.

If linear polarization is incident upon a Brewster face, conventionalmodes such as a TEM₀₀ mode cannot be sustained because losses occur atthe dielectric-air interface which are dependent upon the angle φ shownin FIG. 11. For φ=0 and 360° for example, reflective losses at the inputface will be total (reflectivity R=1), while for φ=90 and 180°,reflective losses will be zero (φ=0). For large apex angles α, however,typically greater than about 45°, this situation can be remedied bycoating the input and exit faces with an anti-reflective dielectriccoating designed to provide the same loss for radial and tangentialpolarization regardless of the value of φ.

With the TIR rod amplifier described here, however, an effect occursthat does not occur in conventional straight-through rod or slabamplifiers. In general depolarization occurs when a ray is incident upona dielectric surface at an angle greater than the TIR angle. FIG. 12shows a ray incident on the TIR rod face that is reflected and isreflected from the barrel faces. The plane 92 through the TIRreflections 94 and the incident point is referred to as the plane ofincidence; and a ray initially polarized can be broken up into twocomponents, perpendicular to the plane of incidence, and parallel to it.The parallel component is identical to the r component of FIG. 11, andis also referred to as the s component. The perpendicular component isthe same as the theta component in FIG. 11 and is referred to as the pcomponent.

For radial polarization only, there is no theta component and inreflection the radial or s component remains s, that is there is nodepolarization. This situation is the analog of that in the slab laserwhere only an incident s component remains an s component. For incidentlight that is linearly polarized, as may happen for small apex angles αfor example, both s and p components exist upon TIR reflection, and aphase delay δ results. This phase delay depends only upon the angle ofincidence and the index of refraction of the rod material and the mediaadjacent to the rod barrel. Since there is a phase delay δ associatedwith each TIR reflection, after N bounces a total phase delay of Nδresults. In the TIR rod in which the path length of each ray isidentical as is the number of TIR reflections each suffers, the totalphase delay of each ray is identical, irrespective of the enteringposition on the rod conical face. The phase delay δ can be calculatedfrom: ##EQU2## Here n₁₂ =n₁ /n₂, where n₁ is the index of the mediaoutside of the rod barrel and n₂ is the index of the laser rod. See MaxBorn and Emil Wolf, "Principles of Optics," Sixth Edition, PergamonPress (1980). This expression vanishes for an angle of incidence of 90°,and at the critical angle θ_(c), as shown in FIG. 13 where the phasedelay (in degrees) is plotted against the angle of incidence at the TIRrod barrel. The calculation was for Nd or Yb:YAG with index n₁ =1.82 andimmersed in water with index n₂ =1.33. The maximum phase delay of 35.37°occurs for an angle of incidence of about 57°.

For the s and p components to be back "in step" and return to linearpolarization, we must have a total phase delay in the rod of 360degrees. Since the maximum phase delay per reflection is only 35.37degrees, we must have multiple reflections to achieve the condition

    Nδ=2πrad=360°

The previous relationships and FIG. 13 may be used to calculate how manybounces are needed to just satisfy this criteria for various roddiameters and apex angles α. For an apex angle of 41.65°, whichcorresponds to an angle of incidence on the end faces of 48.35°, and anangle of incidence on the barrel of 65.89°, we find that for N=12 (12TIR bounces) and a typical Nd:YAG rod diameter of 5 mm, the rod lengthmust be 13.41 cm. Similarly, for a 2 mm diameter rod typical of thatused for Yb:YAG lasers, for N=12 the rod length is 5.36 cm.

Certain geometries, or numbers or bounces, give complete compensationfor the phase delay between the s and p polarization components,resulting in no net depolarization after passing through the TIR rod.This situation is unique to TIR rod lasers and does not happen in TIRslab lasers. For some laser systems, radial polarization may beacceptable and under that circumstance there are no restrictions on thegeometry of the TIR rod excepting those needed for internal compensationof thermal effects. For linear polarization, the s and p depolarizationvanishes for certain rod geometries. Of particular interest are thosethat use relatively large apex angles or small angles of incidence wheredielectric coatings applied to the end faces can be used to achieve nearequal losses for any position and polarization on the faces. This ismost easily achieved when the apex angle is large. Another goal of alarge apex angle is to minimize depolarization losses due to the endfaces alone.

FIG. 14 shows one implementation of the TIR rod amplifier. The resonator96 is conventional and known as a confocal or semi-symmetric resonator.One end consists of a curved high reflector 98 while the oppositeoutcoupler 100 is a flat mirror or partial reflector, which is allowedto leak out some of the laser light. The resonator 96 is stable in thesense understood by laser designers. The g1 parameter is

    g1=1-L/R.sub.1

and g2 is

    g2=1-L/R.sub.2

where R₁ and R₂ are the radii of curvature of the two mirrors, and L theseparation of them, then the resonator is stable if

    0<g1g2<1

a condition satisfied by the resonator in FIG. 13. Note, however, thatbecause there is no thermal focusing, that g1g2 remains constantindependent of average power and therefore the beam and mode quality ofthe laser are also independent of average power. Conventional rod lasersare very power dependent, with mode content and beam quality changingwith average power, and the resonator must be analyzed by including apower dependent lens in the resonator.

While preferred embodiments of the invention have been set forth forpurposes of illustration, the foregoing description should not be deemeda limitation of the invention herein. Accordingly, variousmodifications, adaptations and alternatives may occur to one skilled inthe art without departing from the spirit and the scope of the presentinvention.

What is claimed is:
 1. A lasing rod composed of crystalline or glassmaterial doped with at least one lasing ion defining an optical axis andhaving an axially extending, substantially optically flat, exteriorsurface and first and second substantially optically flat conical endsurfaces,wherein the first conical end surface defines a convex surfaceand the second conical end surface defines a concave surface.
 2. Thelasing rod of claim 1 wherein the first and second conical end surfacesare coaxial with the optical axis.
 3. The lasing rod of claim 1 whereinthe first and second conical end surfaces are congruent.
 4. The lasingrod of claim 1 wherein the first and second conical end surface eachcomprise an apex having a truncated tip.
 5. The lasing rod of claim 1further comprising an annulus of fluid disposed adjacent the exteriorsurface for actively cooling the rod.
 6. The lasing rod of claim 5wherein the fluid is selected from the group consisting of water, waterand ethylene glycol, gas, and cryogenic fluid.
 7. The lasing rod ofclaim 1 further comprising an axially extending center column and afirst annular portion, the center column being doped with an activelasing ion and the first annular portion being composed of a clear lasermaterial or an amplified spontaneous emission absorbing material.
 8. Thelasing rod of claim 7 wherein the center column is composed of Nd:YAG orYb:YAG.
 9. The lasing rod of claim 7 wherein the first annular portionis composed of YAG.
 10. The lasing rod of claim 9 further comprising asecond annular portion, the first annular portion being disposedintermediate the second annular portion and the center column, thesecond annular portion comprising an amplified spontaneous emissionabsorbing material.
 11. The lasing rod of claim 7 wherein the firstannular portion is composed of YAG doped with Cr⁴⁺.
 12. The lasing rodof claim 7 wherein the first annular portion comprises a plurality ofstrips bonded to the center column.
 13. The lasing rod of claim 1wherein the rod focuses light along the optical axis, the first conicalend surface comprises an apex having an angle α, α having a valueselected to smear the focus along the optical axis.
 14. The lasing rodof claim 1 wherein the first and second conical end surfaces eachcomprise an apex having an angle α, α having a value substantially equalto the complement of Brewster's angle.
 15. The lasing rod of claim 1wherein the rod has a geometry comprising a diameter and a lengthselected to provide substantially no net depolarization of an incidentbeam from TIR reflections.
 16. A laser resonator comprising:a lasing rodcomposed of crystalline or glass material doped with at least one lasingion defining an optical axis and having an axially extending exteriorsurface and first and second conical end surfaces, an outcouplerdisposed adjacent the second conical end surface; and wherein one of thefirst or second end surfaces is convex and the other of the first orsecond end surfaces is concave a high reflector disposed adjacent thefirst conical end surface.
 17. The laser resonator of claim 16 whereinthe outcoupler comprises a partial reflector.
 18. The laser resonator ofclaim 16 wherein the high reflector has a face disposed adjacent thefirst conical end surface, the face of the high reflector having aconcave surface.
 19. The laser resonator of claim 16 wherein the firstand second conical end surfaces are congruent.